1. Field of the Invention
This invention relates to a method for fabricating a resistor for a resistance random access memory (RRAM), more particularly to a method involving forming a variable resistance layer of zirconium oxide on an electrode under a working temperature ranging from 175° C. to 225° C.
2. Description of the Related Art
Generally, a resistance random access memory (RRAM) circuit is composed of an array of 1T1R cross point memory cells (as described in U.S. Pat. No. 7,208,372), each of which has a transistor and a resistor, or composed of an array of 1D1R cross point memory cells, each of which has a diode and a resistor.
The resistor has a tri-layer structure including a top electrode, a bottom electrode, and an insulating layer sandwiched between the top and bottom electrodes. Transition metal oxides, such as NiO, CuO, ZrO2, TiO2 and HfO2, with a variable resistance are widely used as the insulating layer. The aforesaid transition metal oxides exhibit a property that the resistance thereof can be switched between a high resistance state (which can be referred as OFF-state) and a low resistance state (which can be referred as ON-state) by applying a set (a write action) or a reset (an erasing action) voltage to the resistor. Several mechanisms of resistive switching between ON-state and OFF-state for the resistors have been proposed over the years (as described in U.S. Patent Application Publication No. 2007/0269683). The high and low resistance states of the insulating layer within the resistor can be used to define as a two-state information (0, 1) stored in the RRAM circuit, and can be read by applying a reading voltage to the resistor.
A conventional method for fabricating a resistor for a resistance random access memory (RRAM) (see IEEE ELECTRON DEVICE LETTERS, VOL. 28, NO. 5, PP. 366˜368, MAY 2007 by the inventors of the present application) includes the following steps: (A) forming a SiO2 layer on a Si substrate; (B) forming a bottom electrode of a layer structure of Pt/Ti on the SiO2 layer; (C) forming a ZrO2 insulator layer having a layer thickness of 70 nm on the bottom electrode under a working temperature of 250° C. by using a radio-frequency magnetron sputtering system (not shown); and (D) forming a Ti layer, which serves as a top electrode on the ZrO2 insulator layer, by using the radio-frequency magnetron sputtering system.
Since the formation of the resistors comes after the formation of the transistors (or the diodes) during a process of fabricating the resistance random access memory (RRAM) circuit, the working temperature (250° C.) when forming the ZrO2 insulator layer is too high for the fabricated transistors (or the diodes), and would cause defects in the transistors (or the diodes). Furthermore, the life cycle of the resistor is also needed to be enhanced, i.e., more in number of resistive switching times between ON-state and OFF-state that the resistor can endure.